THE RACING LINE

Tyres, the Basics

So what is a tyre? Basically its a band of rubber material around the outside of your wheel that provides friction between the ground and your chassis to stop it sliding all over the place.

In racing terms, the tyres are usually smooth with no tread, except for rain tyres which have grooves in them to allow water to channel away.

Racing tyres are made of a mixture of Rubber, Carbon Black, Sulphur, Oil and additives and this makes up its "Compound". Slight variations in the mixture produce different compounds that provide more grip or longer life. Harder compounds last longer but do not provide a lot of grip. Softer compounds increase the grip, but don't last as long.

So what makes one tyre compound grip more than another?

Think of a plastic block. When it is cold and unheated, it is fairly strong. Dragging it along sandpaper will scratch it, but not remove much material from it. It also requires less effort to pull it along the sandpaper.

Now, imagine that the block is heated enough to make is soft and malable. Dragging it along the sandpaper pulls more material off of the block but it sticks to the sandpaper more and requires more effort to pull it a long.

If we look at this in the context of a tyre we can see how compounds work.

When a tyre is in contact with the ground, the friction causes the tyre to heat up. If the tyre is worked hard or badly set-up, the compound gets soft and grips the track.

Great!

However, if it gets too hot, its gets over sticky and compound gets pulled off the tyre as it sticks to the track. This means that although you get grip for a few laps, the tyre quickly degradeds and you have to come in for fresh rubber. Also, over sticky tyres can cause an effect similar to swimming in treacle. The tyres stick to the ground so much that you lose speed on the straights as the tyres resist rolling.

If the tyres do overheat, due to driving style or high ambient temperature, a harder compound may be used. This harder compound takes more effort to warm up and therefore can be worked just as hard, without getting too soft.

Sadly, there are usually only three choices of compound - Hard, Medium or Soft - and often the next one up is too hard and although it lasts the distance, it doesn't offer as much grip as the softer one did for those few laps.

With Indycar and Nascar having an option to alter the weather, this can add a new dimemsion to your tyre choice

On cold days, it can be dificult to get heat into the tyres to soften them up. This can affect your laptimes because you don't get much grip. On a very hot day, you tyres can often get too soft and stick causing them to wear faster and if too sticky, cost you time.

Tyre Grip

How much grip a tyre gives depends on a few things - It's compound, it's size and the weight it carries.

Heres an experiment to try. If you can, use the spare tyre from your/your parents car or an old one that you might be able to find.

First of all we need to know the weight of the tyre so weigh it. Next, go outside and place the tyre on the ground and, towards the base of the tyre, push against it sideways until it moves. Next try the same experiment on a slippy surface or, if you can, make the ground greasy with some water and detergent. The force you need to apply to make the tyre move should be less.

To get an idea of whats going on we need to measure the amount of force you need to move the tyre. A good way of doing this is to place a set of bathroom scales between you and the tyre when pushing. This isn't very accurate, but it gives an idea. The weight on the scales will increase and should reach a maximum before the tyre slips. This is the figure you need. Repeat the process for the slippy surface.

Now, lets say that on the dry surface you needed to apply 40kg of force to move the tyre and the tyre weighs 25kg. By dividing the force needed by the tyres weight we get its grip level in "gees".

So, for our dry surface we get:

40/25 = 1.60G

If we say it took 28kg of force to move the tyre on our slippery surface we get:

28/25 = 1.12G

Now, don't get excited! In a static test you may of found that you tyre can hold a 1.60G manouvre but in drving conditions there are a number of factors that will reduce this. When driving, a figure of about 0.90G - 1.00G is reasonable is very good.

For the second part of this experiment, we need to make the tyre heavier. Try slinging some balast around the center of the tyre. Now my tyre weighs 40kg and the force needed to move it on a dry surface is 64kg.

Using our maths, we get:

64/40 = 1.60G

So, we are still getting a grip level of 1.60G. The grip level we obtain in "gees" is called the coefficient of adhesion

We have now experienced and observed the fundemental law of adhesion:

"The force needed to slide a tyre is proportional to the weight supported by the tyre"

The force required is called the adhesive limit. The equation to express this is F <= µW where F is the force required, µ is the coefficient of adhesion and W is the weight or load on the contact patch. F and Wshare the same units so µ is just a number, or propotionality constant.

Now lets look at our racing car. It weighs 1500kg and the coefficient of adhesion is 1.60G. Using F = µW we get:

1.60 X 1500 = 2400

This means that to push your racing car, sideways down the track, you'd need to push with 2400kg of force or around 5200lbs!

Over the limit

So how hard can we push the tyres on the car?

Remember our racing car? It weighs 1500kg and has even weight distribution of 50 - 50. It's track is 150cm.

Let's imagine that we throw our car into a 1G cornering manoeuvre. Referring to lesson one, we can calculate the weight transfer to each side of the car.

If the manoeuvre is 1G, we can say that the cornering force is 1500kg. If we call the height of the Centre of Gravity h, track t, weight G, cornering force f and static weight distribution d expressed as a fraction of weight to the left, we can use:

fo = dG + fh/t

fi = (1 - d)G - fh/t

So we get:

0.5 X 1500 + (1500 X 50)/150 = 1250kg

(1 - 0.5) X 1500 + (1500 X 50)/150 = 250kg

With this we can see that this would transfer 1250kg of weight to the outside and leave 250kg on the inside. Lets say that the tyres give, when cold, a coefficient of adhesion of 0.90G.

Using our formula we can see that for the outside edge:

0.90 X 1250 = 1125

And for the inside we get:

0.90 X 250 = 225

So, adding these together, we get a total force of 1350kg needed to make the tyres unstick and the car slip. Because we are pulling a 1G manoeuvre in a 1500kg car, we are applying 150kg of excess force to the tyres. The inside tyres only need 225kg of force so they break away first causing the car to judder and lose all grip. Having lost grip on two tyres the, outside tyres now have to support the entire force of the turn. This puts 1500kg onto the outside tyres which can only hold 1125kg of force. So they then lose their grip. Voila, the car slides off into the scenery.

Now normally this sudden application of 1G to the car would probably only happen in a spin where the car yaws and the direction of force shifts from the centre of gravity to the front, to the centre of gravity to the side. If you think about it, when you see a car spin it starts to go and then snaps away (more on this in another lesson)

Under normal cornering, the force builds up. You may enter the corner at a say, 0.20G and at maximum you reach 0.80G.

Keep in your minds the fact that in a corner, weight is transferred from the inside edge to the outside edge.

Now, using the formula from lesson one (and remembering to replace the wheelbase with the track) we can work out the maximum cornering "gees" for the car.

If we pull a 0.2G corner, the weight transfer is 850kg to the outside edge and the inside becomes 650kg. With this we can calculate the forces needed to make the tyres slip with F = µW

0.90 X 850 = 765kg for the outside edge.

0.90 X 650 = 585kg for the inside edge.

Now, we're pulling a 0.2G manoeuvre so the total force applied to the car is 0.2 X 1500kg which gives us 300kg. This figure isn't enough to make the inside or outside tyres slip so we're O.K.

Lets take a look for a 0.8G manoeuvre, again using the same formula. The weight transfer this time is 1150kg to the outside and 350kg on the inside. The forces needed to make it slip this time are:

0.90 X 1150 = 1035kg for the outside edge.

0.90 X 350 = 315kg for the inside edge.

This time our applied force is 0.8 X 1500 which is 1200kg. This time we are applying too much force to all the tyres and, again, we are over the limit and are off farming.

With a bit of work we can work out what the maximum cornering "gees" are.

Sadly, I must admit that I'm no genius and I can't derive a formula to calculate the maximum, however, if somebody would like to, I'll include it in this lesson and credit them for it!

Another point to remember is that each corner will have different amounts of grip. If you accelerate into a bend, you tend to understeer because apart from weight transfer to the side, it is also transferred to the rear. When braking, the opposite happens, and the car tends to oversteer. We'll look at this more when we look at set-ups.